Snapping Screws On An Electric Molding Machine…

I was at a customers facility the other day and they had a problem which is becoming more common with today’s electric molding machines…

We bought these fancy, new, direct-drive electric molding machines. Overall, we like them, but we have already snapped three screws and the manufacturer has no idea why this is happening. Do you have any ideas?
Note: After further discussion and a review of their processes, I determined that they were not using sufficient screw delay prior to recovery… In some cases they were not using any screw delay at all.
My Response
Immediately after 2nd stage pack, there is typically a large amount of pressure present in front of the screw. If this pressure is not relieved, it will increase the torque applied during screw rotation. This additional force can quickly weaken, twist, or even break the screw within the barrel when screw recovery starts.
This concern is even greater with electric injection molding machines, since the servo-motor can apply a great amount of torque instantaneously…. especially on direct-drive machines where the minute flex of the belt is non-existent.
To relieve the pressure that’s at the front of screw and to prevent unnecessary screw damage, the ‘screw delay’ or ‘screw decompression’ option should be used. The screw delay option allows you to add a delay after 2nd stage packing to relieve the plastic pressure on the screw before recovery. The decompression will actually back up the screw to relieve the pressure… similar to decompression after recovery.
Additional Thoughts
Hydraulic molding machines often avoid this issue since they have an inherent buffer due to the slight buildup of pressure as the valve opens, and a period of pressure stabilization that occurs before the screw reaches full speed. Adversely, the electric servo-motors apply a high amount of torque to the screw virtually instantaneously. People who are are familiar with hydraulic molding machines often have difficulty adjusting to the instantaneous response of an all-electric molding machine.

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